1. Field of the Invention
The present invention relates to an X-ray projection exposure apparatus, and more particularly, to an X-ray projection exposure apparatus which is suitable for transferring a circuit pattern formed on a mask (also referred to as "reticle") onto a substrate, such as a wafer, via a reflective type focusing X-ray optical system using a mirror projection scheme or the like.
2. Discussion of the Related Art
Conventionally, in exposure apparatus used for semiconductor manufacture, circuit patterns formed on a mask (photo-mask) used as an object surface are projected and transferred onto the surface of a photosensitive substrate such as a wafer or substrate for forming a mask, etc., via a focusing optical system. The photosensitive substrate is coated with a resist. The resist is exposed with exposing light to form a rest pattern.
The resolving power W of the exposure apparatus is determined mainly by the wavelength .lambda. of the exposing light and the numerical aperture NA of the focusing optical system, and is expressed by the following equation: EQU W=k1.lambda./NA (k1:constant) (1)
Accordingly, in order to improve the resolving power, it is necessary to shorten the wavelength and/or increase the numerical aperture. Currently, exposure apparatus used in the manufacture of semiconductor devices uses mainly the i-line having a wavelength of 365 nm, and a resolving power of 0.5 .mu.m is obtained at a numerical aperture of about 0.5. Since increasing the numerical aperture is difficult due to various constraints in optical design, it will be necessary in the future to shorten the wavelength of the exposing light. Excimer lasers are examples of exposing light that has a wavelength shorter than the i-line. The wavelengths are 248 nm for the KrF excimer laser and 193 for the ArF excimer laser, respectively. A resolving power of 0.25 .mu.m is obtained in the case of the KrF excimer laser, and a resolving power of 0.18 .mu.m is obtained in the case of ArF. Furthermore, if X-rays with an even shorter wavelength are used as exposing light, a resolving power of 0.1 .mu.m or less should be possible at a wavelength of 13 nm, for example.
The main components of the conventional exposure apparatus are a light source, an illumination optical system, and a projection focusing optical system. The projection focusing optical system is constructed from a plurality of lenses or reflective mirrors, etc., so as to focus the mask pattern on the mask onto a substrate, such as a wafer.
To obtain a desired resolving power, it is necessary that at least the focusing optical system be essentially free from aberration. If aberration is present in the focusing optical system, the sectional profile of the resist pattern deteriorates, and as a result, adverse effects on the processes following the exposure and/or the problem of image distortion may arise.
In the conventional exposure apparatus for manufacturing semiconductor devices or the like, a position detection device (also referred to as "alignment device") is provided so that a resist pattern can be formed at a predetermined position on the wafer with respect to an existing circuit patterns on the wafer. The alignment device detects the positions of the mask and wafer, and the respective detected positions of the wafer and the mask are adjusted by a wafer stage and a mask stage so that a reduced image of the mask pattern is focused at a prescribed position on the wafer.
An example of the alignment device is an optical detection device. This type of device detects alignment marks on the wafer by illuminating the marks and detecting the light reflected from the alignment marks through a photo-detector, for example. When the wafer position changes, the signal output from the photo-detector also changes, thereby enabling the detection of the wafer position. Similarly, the position of the mask can be detected by illuminating the alignment marks on the mask with illuminating light, and then detecting the light reflected from the alignment marks through a photo-detector, for example.
Such an alignment device can detect the positions of the respective marks on the wafer and the mask with high accuracy. Accordingly, alignment of the mask with respect to the wafer can accurately be performed. In the conventional exposure apparatus, the alignment devices are disposed between the focusing optical system and the wafer and between the focusing optical system and the mask.
Furthermore, in the conventional exposure apparatus, a high resolving power can be obtained in the vicinity of the focal position of the projection focusing optical system. Accordingly, the position of the surface of the wafer that is being exposed must be located in the vicinity of the focal position of the projection focusing optical system. The range in the direction of the optical axis in which the projection focusing optical system exhibits a high resolving power is called the "depth of focus(DOF)." The depth of focus, DOF, is determined mainly by the wavelength .lambda. of the exposing light and the numerical aperture NA of the focusing optical system, and is expressed by the following equation: EQU DOF=k2.lambda./NA.sup.2 (k2:constant) (2)
For example, if the numerical aperture is 0.5 and the constant K2 is 1 at a wavelength of 365 nm, then the DOF is 1.5 .mu.m.
In order to expose the wafer surface while the wafer surface is positioned within the range of the depth of focus, a device for detecting the position of the wafer surface in the direction of the optical axis of the projection focusing optical system (also referred to as "focal point detection device," because the device detects the vertical position of the wafer in order to position the wafer at the focal point) is installed in the exposure apparatus. Through this device, the position of the wafer in the direction of the optical axis is detected, and the position of the wafer in the direction of the optical axis is adjusted by the wafer stage so as to position the wafer surface at an appropriate position within the DOF.
FIG. 12 schematically shows an example of such a focal point detection device. The detection scheme illustrated in FIG. 12 is generally referred to as the triangulation method. In this method, wafer 6 is illuminated with illuminating light 91, which is obliquely incident on the wafer 6 through mirror 95, and light 92 reflected from the wafer is detected by a photo-detector 96 through mirror 95. When the wafer position changes, the optical path of the reflected light changes, which in turn changes the position of the reflected light at the photo-detector 96. Thus, by detecting such position changes at the photo-detector 96, the position of the wafer can be measured. A one-dimensional or two-dimensional position detection sensor is used as the photo-detector.
Such a focal point detection device is advantageous because the position on the wafer at which the focal point detection device detects the position of the surface (i.e., the position illuminated by detection light) can be set inside the area being exposed or in the vicinity thereof. In the conventional exposure apparatus, the focal point detection device is installed between the focusing optical system and the wafer.
FIGS. 13 and 14 are schematic diagrams illustrating examples of conventional exposure apparatus that uses the i- line. This apparatus is constructed mainly from a light source and illumination optical system (not shown in the figures), a stage 15 for holding mask 14, a projection focusing optical system 13, a stage 17 for holding wafer 16, alignment devices 18 and 18' (FIG. 13), and a focal point detection system 18" (FIG. 14). The mask 14 has a mask pattern thereon, which is to be transferred onto the wafer 16 without reduction or with a certain reduction factor. The projection focusing optical system 13 is constructed of a plurality of lenses, etc., in such a way as to focus the image of the mask pattern on the mask 14 onto the wafer 16. The focusing optical system 13 has a field of view, the diameter of which is about 20 mm, and is constructed in such a way as to transfer the mask pattern onto the wafer 16 at once. The alignment detection devices 18 and 18' detects the positions of respective alignment marks on the mask and the wafer. The focal point detection system 18" emits a light beam 91, such as visible light beam, towards the wafer 16 obliquely, and detects the light beam 92 reflected from the surface of the wafer 16.
In the conventional exposure apparatus using the i-line or the like, as described above, the projection focusing optical system can be constructed of leases. Accordingly, an optical system with a field of view of 20 mm square or larger can be designed. Thus, it has been possible to expose a desired region (e.g., a region corresponding to two (2) semiconductor chips) at once.
On the other hand, in designing a focusing optical system for X-rays in an effort to obtain a higher resolving power, it is found that the field of view needs to be reduced. Therefore, an exposure region as large as that in the above-mentioned exposure apparatus cannot be exposed at once. Accordingly, a scanning method has been proposed. In the scanning method, a semiconductor chip area of 20 mm square or larger can be exposed using a focusing optical system having a small field of view by synchronously scanning the mask and the wafer during exposure. Using such a method, it is possible to expose the desired exposure region by an X-ray projection exposure apparatus.
For example, in the case of exposure by X-rays having a wavelength of 13 nm, it is possible to form the exposure field of view of the projection focusing optical system to be an annular band shape so that a high resolving power can be obtained.
FIGS. 15 and 16 schematically show examples of proposed designs of X-ray projection exposure apparatus. The X-ray projection exposure apparatus includes an X-ray source 1, a X-ray illumination optical system 2, a stage 5 for holding a mask 4, an X-ray projection focusing optical system 3, and a stage 7 for holding a wafer 6. The mask 4 has a mask pattern thereon, which is to be transferred onto the wafer 6 without reduction or with a certain reduction factor. The projection focusing optical system 3 includes a plurality of reflective mirrors 31-34, etc., in such a way as to focus the mask pattern on the wafer 6. The focusing optical system has an annular band shape field of view so as to transfer a portion of the mask pattern on the mask 4 having an annular band shape onto the wafer 6. During exposure, the mask 4 is illuminated with X-rays 91, and the reflected X-rays 92 are guided towards the wafer 6 via the X-ray projection focusing optical system 3. The mask 4 and wafer 6 are synchronously scanned with the X-rays at respective constant speeds to expose an entire predetermined region (e.g., a region corresponding to one semiconductor chip).
In this example of X-ray projection exposure apparatus, due to various constraints to the design of X-ray optical systems, the reflective mirror closest to the wafer (mirror 34 in FIG. 15 or mirror 31 in FIG. 16) needs to be disposed in proximity to the wafer. Accordingly, it is difficult to install the optical systems for alignment devices and the focal position detection device between the focusing optical system and the wafer. In this connection, the following two problems are worth noting. (1) If the position of the reflective mirror closest to the wafer among the reflective mirrors of the focusing optical system is removed from the wafer in order to increase the gap between the wafer and the closest reflective mirrors, the focusing performance of the focusing optical system suffers, and as a result, the desired pattern with a sufficient resolution cannot be obtained. (2) If the reflective mirror closest to the wafer among the reflective mirrors of the focusing optical system is made thinner in order to increase the gap, the rigidity of such a mirror drops, making manufacture of a high-precision mirror difficult. Thus, in the design of the X-ray exposure apparatus, it is difficult to increase the gap between the mirror closest to the wafer and the wafer without sacrificing the optical characteristics of the focusing optical system.
Furthermore, because no operational X-ray projection apparatus has yet been developed, there is no specific proposals as to the arrangement of the above-mentioned focal point detection system (system for detecting the position of the wafer surface in the optical axis direction).